Typically defined as an online broadcast, webcasts are usually a one-way flow of information to a large audience. Similar to your local television network, the purpose of a webcast is to distribute a video or audio signal to a large amount of people. They are normally streamed live via the internet or can be accessed on-demand.
Webinars are usually online interactive meetings, with content typically focused on education or training. The audience size of a webinar is also normally smaller than that of a webcast, and often engages the audience with polling, Q&A or social media.
Your event is just a snap shot in time and is soon over. It would be a shame for all that valuable content to be lost or forgotten the moment delegates leave the conference room. Quite often people are unable to attend and would appreciate the opportunity to see the presentations in their own time or view edited highlights. Do you need to promote your event next year or provide a motivational video post-event?